In one of the methods of mounting electronic components such as bare IC chips on a substrate, the electronic component is held on a head while a substrate is set on a stage, and after the electronic component is positioned relative to the substrate, the head is lowered so that a bump on the electronic component makes contact with a corresponding electrode pad on the substrate, to bond the bump of the electronic component and the electrode pad of the substrate. In one bonding method, a certain load is applied between a gold bump provided on the electronic component and an electrode pad on the substrate, and ultrasonic energy with or without thermal energy is applied to bond the component on the substrate . In another method, solder bumps are provided on the electronic component, which are reflowed by applying thermal energy, after the bumps are brought into contact with electrode pads on the substrate, such that the bump is bonded to the pad by solder.
As the circuit wiring pitch in circuits of the electronic component is now in the order of nanometers, low dielectric constant materials are used for the interlayer insulating film. One problem with the low dielectric constant material is that it has low strength and may crack when subjected to a large load in the bonding process, which may lead to breakage of the electronic component. The load applied to the electronic component when its bump makes contact with the electrode pad on the substrate (hereinafter, referred to as contact load) in the process of lowering the head holding the electronic component onto the substrate can readily be too large, and this load poses a risk of damage to the electronic component using the above-noted low dielectric constant material because of a crack in the interlayer insulating film.
Now, an electronic component mounting method with a local reflow process using solder will be described with reference to FIG. 5A to FIG. 5D. In this method, discrete electronic components with solder bumps are mounted on a substrate, after which the electronic components are heated using a head to reflow and bond the solder bumps with electrode pads on the substrate.
Referring to FIG. 5A to FIG. 5D, an electronic component 31 having solder bumps 32 is held by a tool 30 on a head. A substrate 33 having electrode pads 34 on which solder or flux has been applied is carried in and set on a stage 35. The head is moved to position the solder bumps 32 of the electronic component 31 relative to the electrode pads 34 on the substrate 33, as shown in FIG. 5A. The head is lowered until the solder bumps 32 make contact with the electrode pads 34 as shown in FIG. 5B. The electronic component 31 is then heated with a heater disposed at the lower end of the head to reflow the solder bumps 32 so that the electrodes on the electronic component and the electrode pads 34 are bonded by the solder bonds 36, as shown in FIG. 5C. Then, the heating is stopped and the component is cooled to set the solder bonds 36, after which the head releases the electronic component 31 and goes up, as shown in FIG. 5D. After that, the substrate 33 on which the electronic component 31 is mounted is carried out from the stage 35.
FIG. 6 and FIG. 7A to FIG. 7C illustrate how the head is controlled in the process of lowering it until the component 31 (the solder bumps 32) make contact with the substrate 33 (the electrode pads 34) in this electronic component mounting method. The head is lowered at high speed from a predetermined waiting position to a slow down starting position, which is set at a position where there is no risk of accidental contact between the component and the substrate. From there the head is lowered at a low search speed of about 0.1 mm/s. The head has a build-in load cell for measuring the load in real time, and it is determined whether or not the load has reached a predetermined detection level of contact load. The lowering movement of the head is continued at the low speed until the load reaches the detection level, when it is slowed down and stopped. This way, the component is brought into contact with the substrate in a shortest possible time without the risk of applying an impact load when the component touches the substrate. After the electronic component makes contact with the substrate, the mounting head is controlled to repeat the steps of moving slightly and measuring the load, so that a predetermined load is applied to them (see Japanese Patent Laid-Open Publication No. 2003-8196).
In this process of bringing the component into contact with the substrate, however, as shown in FIG. 7C, the head is slowed down at a time point E, which is delayed by d from a time point D when a detection level contact load is detected. There is a time delay of e from the time point E to F when the head is stopped, as a result of which the contact load at the stop time point F is much larger than the contact load that was first detected. To be more specific, if a detection level contact load of 0.5 N is detected while the head is moving at a search speed of 0.1 mm/s, the load when the head is stopped will be as large as 2.0 to 3.0 N. Moreover, the load further increases and reaches a peak immediately after the head is stopped by the inertia of the head in a period of about 5 to 10 msec as indicated by an imaginary line in FIG. 7C. This large, instantly applied load may generate cracks in the interlayer insulating film made of low dielectric constant material and cause damage to the electronic component.
An object of the present invention is to solve the above-described problems in the conventional technique and to provide a method of controlling contact load in an electronic component mounting apparatus, with which the contact load applied on the components is precisely controlled to be as close as possible to a predetermined target contact load of a low level, so that electronic components using a low dielectric constant material can be mounted without the risk of damage.